From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752965AbcB2RdQ (ORCPT ); Mon, 29 Feb 2016 12:33:16 -0500 Received: from mail-wm0-f51.google.com ([74.125.82.51]:33941 "EHLO mail-wm0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751392AbcB2RdP (ORCPT ); Mon, 29 Feb 2016 12:33:15 -0500 Date: Mon, 29 Feb 2016 18:33:12 +0100 From: Michal Hocko To: "Shi, Yang" Cc: tj@kernel.org, jack@suse.cz, axboe@fb.com, fengguang.wu@intel.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linaro-kernel@lists.linaro.org Subject: Re: [RFC PATCH] writeback: move list_lock down into the for loop Message-ID: <20160229173312.GK16930@dhcp22.suse.cz> References: <1456505185-21566-1-git-send-email-yang.shi@linaro.org> <20160229150618.GA16939@dhcp22.suse.cz> <56D47F90.9050903@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56D47F90.9050903@linaro.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 29-02-16 09:27:44, Shi, Yang wrote: > On 2/29/2016 7:06 AM, Michal Hocko wrote: > >On Fri 26-02-16 08:46:25, Yang Shi wrote: > >>The list_lock was moved outside the for loop by commit > >>e8dfc30582995ae12454cda517b17d6294175b07 ("writeback: elevate queue_io() > >>into wb_writeback())", however, the commit log says "No behavior change", so > >>it sounds safe to have the list_lock acquired inside the for loop as it did > >>before. > >>Leave tracepoints outside the critical area since tracepoints already have > >>preempt disabled. > > > >The patch says what but it completely misses the why part. > > I'm just wondering the finer grained lock may reach a little better > performance, i.e. more likely for preempt, lower latency. If this is supposed to be a performance enhancement then some numbers would definitely make it easier to get in. Or even an arguments to back your theory. Basing your argument on 4+ years commit doesn't really seem sound... Just to make it clear, I am not opposing the patch I just stumbled over it and the changelog was just too terrible which made me response. -- Michal Hocko SUSE Labs